Cadomian vs. Variscan evolution of the Ossa-Morena zone (SW Iberia): field and 40Ar/ 39Ar mineral age constraints

Abstract

Six hornblende and ten muscovite concentrates have been dated from three contrasting tectonic units exposed within the Ossa-Morena zone of the Iberian Massif. These include: (1) the Obejo-Valsequillo domain (north of the Badajoz-Córdoba shear zone); (2) the Sierra Albarrana structural unit (immediately south of the Badajoz-Córdoba shear zone); and (3) the thermal some exposed within the Olivenza-Monesterio antiform (Zafra-Monesterio domain). Hornblende from foliated amphibolite in the Obejo-Valsequillo domain (Siera Negra unit) displays internally discordant 40Ar/ 39Ar age spectra as a result of both intracrystalline contamination with extraneous argon and slight rejuvenation. 36Ar/ 40Ar vs. 39Ar/ 40Ar isotope-correlations are generally well-defined and yield ages of c. 550–560 Ma. These are interpreted to date cooling following late Precambrian-early Paleozoic (Cadomian) tectonothermal activity. This was followed by slight Variscan rejuvenation at c. 375–400 Ma. Muscovite from this unit displays internally discordant age spectra which reflect more extensive Variscan rejuvenation of intracrystalline argon systems which had initially cooled below appropriate blocking temperatures at c. 560 Ma. Hornblende and muscovite from the Sierra Albarrana domain (Sierra Albarrana Group) record 40Ar/ 39Ar plateau and isotope-correlation ages which range between c. 392 Ma and 351 Ma. These results indicate that complete Variscan rejuvenation was followed by relatively rapid post-metamorphic cooling. Hornblende separated from various lithologie elements exposed in the Olivenza-Monesterio Antiform yield variable results. Hornblende within a mafic xenolith in the Monesterio Granodiorite records an isotope-correlation age of c. 553 Ma which is interpreted to date the last cooling through appropriate argon retention temperatures. Hornblende within amphibolite of the Montemolin Series (lower Serie Negra) records internally discordant age spectra which reflect extensive Variscan (c. 390–400 Ma) rejuvenation of intracrystalline argon systems which had initially recorded ages > c. 500 Ma. Muscovite was concentrated from several lithologie elements in this area, including: (1) mylonitic Monesterio Granodiorite; (2) metasedimentary rocks of the Montemolin succession (displaying high-grade, sillimanite-potassium feldspar-bearing assemblages); and (3) migmatitic schist of the Siere Negra Group. These muscovite concentrates display similarly discordant 40Ar/ 39Ar age spectra which suggest significant but incomplete Variscan rejuvenation occurred at c. 400 Ma. This appears to have affected intracrystalline argon systems which had initially cooled through appropriate closure temperatures sometime prior to c. 450 Ma following an initial high-grade (Cadomian?) metamorphism. The 40Ar/ 39Ar results clearly reflect a variable, complex, and polymetamorphic evolution for the Ossa-Morena zone. Internal consistency of data within each regional tectonic unit compared with marked contrast between the units provides evidence of the extremely heterogeneous Variscan tectonothermal overprint. The results demonstrate that the present tectonic architecture of the Ossa-Morena Zone was not solely the result of late Variscan wrench tectonics, but, instead, a consequence of processes active since at least the Lower-Middle Devonian. Juxtaposition of regional structural units with contrasting tectonothermal histories, and recording variable rates and extents of uplift are consistent with maintainence of an overall transpressional Variscan tectonic regime.